If pubic lice are not the sort of thing you want to be seen reading about, let me give you the opportunity to close your browser window right now. But if you’re at all curious about the secret that pubic lice have been keeping for over three million years, the tale of a mysterious liaison between our ancestors and the ancestors of gorillas–read on.

Many parasites tend to stick close to their hosts. A parasitic wasp may wander through forests and fields to find a caterpillar from a single species of butterfly in which it will lay its eggs. Blood flukes taste the water of their ponds for molecules from human skin. Wolbachia, a species of bacteria, never even has to leave its hosts, because it is passed down from mothers to their offspring. If a parasite sticks to its host for millions of years, their evolution may run on parallel tracks. As the host species splits in two, its parasite splits as well.

One of the best studied cases of parasites and hosts coevolving this way comes from pocket gophers and their lice. Lice do not have wings, and their legs are better suited to gripping hairs than walking. As a result, the only way they can move from a pocket gopher to another host is during close contact. But pocket gophers are an unsocial bunch, generally living alone in their tunnels. What little contact they do make is with other members of their own species–their mates and their offspring. So the lice have few opportunities to meet new species. Even if they do end up on another species, they’re unlikely to survive long. Their adaptations are finely tuned to their traditional hosts. The hairs of pocket gophers vary in size from species to species, for example, and their lice clamp onto their hair with mandibles that are just the right width.

As a result, the evolutionary tree of pocket gopher species looks a lot like the evolutionary tree of their lice. (Check out the figure in this 2004 review: pdf.) There are a few cases in which the branches of the trees are not mirror reflections. There are a few ways in which parasites break the symmetry. Sometimes a parasite diverges into two new species within a single host. Sometimes a parasite species becomes extinct. And sometimes parasites shift to a new host species. The most spectacular example of such a leap occurred within the last century, when a virus moved from chimpanzees to humans, giving rise to the scourge we now know as HIV. (I go into more detail about cospeciation in my book Parasite Rex.)

But back to lice. There are 5,000 known species of lice and probably many more. Most species of birds and mammals are hosts to lice, which are adapted for living on feathers and fur. Like other mammals, primates carry lice, and typically a species of lice can only survive on one species of primate. Their evolutionary trees are very similar, reflecting millions of years of coevolution. But there are some surprises lurking in the trees. As I wrote here, human head lice have a peculiar evolutionary history. According to one study, head lice form two genetically distinct populations that share a common ancestor over a million years ago. One of those groups is most common in Native Americans and other New World groups.

The authors–David Reed of the Florida Museum of Natural History and his colleagues–argue that their history reflects the history of our hominid ancestors. Paleoanthropologists generally agree that hominids originally evolved in Africa, and that several waves of hominids moved to Europe and Asia. Modern humans, however, descend from the hominids that stayed behind in Africa, evolving about 200,000 years ago, and spreading out to the other continents much later. Reed and his colleagues argue that one population of lice remained in Africa on our ancestors. The other population rode on top of the migrating hominids, which eventually evolved into other species such as Homo erectus and Neanderthals. Reed and his colleagues argued that humans must have made contact with Asian hominids some time in the last 100,000 years, whereupon they picked up the second population of lice. (Click on the figure to get a bigger view.)

With this study behind them, the scientists then naturally turned their gaze downward. Along with head lice (Pediculus humanus), humans also have pubic lice (Pthirus pubis). Pubic lice–a k a crabs–usually spread from person to person through sexual contact. They lay their eggs in the pubic hair and grow into adults, feeding on skin and blood. And they only live on us. You can’t get crabs from any animal.

Like their common name suggests, pubic lice are crab-shaped, with wide-reaching legs that help it to move between widely-spaced hairs. They look very different from our lemon-shaped head lice. In fact, early entomologists put human head lice and human pubic lice in two separate genera, grouping each with lice from other apes. Pubic lice, in fact, look a lot more like lice found on gorillas than they look like human head lice.

The scientists set out to recover the evolutionary tree of pubic lice, just as they had done with head lice. They analyzed DNA from human head lice, human pubic lice, as well as other species from the same genera that live on chimpanzees and gorillas. They also analyzed DNA from lice that live on monkeys and on rodents so that they could get a better sense of how pubic lice had evolved from a common ancestor with other species. The scientists not only drew branches for each species, but also estimated when those branches split over the course of history.

Their conclusion, which they published today in BMC Biology, is just as striking as their earlier one about head lice. But it is hardly the same. We did not get pubic lice from other hominids. We got them from the ancestors of gorillas.

The tree, which I’ve reproduced at the bottom of this post, tells the story. Thirteen million years ago, a distant ancestor of today’s great apes was infested with lice. The lice evolved into two species that continued to live on the same host. One of those lice species was the ancestor of today’s Pediculus (which includes head lice). The other species was the ancestor of Pthirus (pubic lice). About seven million years ago the ancestors of gorillas split off from the ancestors of humans, chimpanzees, and bonobos. Both ape lineages carried the Pediculus and Pthirus lice. But then, Reed and his colleagues argue, two extinctions took place. The Pediculus lice became extinct in the ancestors of gorillas. The Pthirus lice became extinct in the human/chimp lineage.

Millions of years passed. The ancestors of humans and chimpanzees split, and their lice evolved into distinct–but similar–species, including what we now know as head lice. And then, about three and a half million years ago, our hominid ancestors (perhaps Lucy?) picked up pubic lice from the ancestors of gorillas.

Is this evidence of a Pliocene love that dare not speak its name? Not according to Reed. He and his colleagues suggest that hominids might have gotten crabs by eating gorilla flesh, perhaps scavenging a carcass. Or they might have slept at nesting sites that gorillas contaminated with their lice. This study just so happens to have come out a few months after another team of scientists showed that chimpanzees not only gave humans HIV but also gave gorillas a related strain of the virus. If chimpanzees can give gorillas a blood-borne virus, it’s not too surprising that gorillas could give hominids some lice.

But there are a couple intriguing implications this study raises. One is that gorillas and hominids lived close together. The fossil record hasn’t had a whole lot to say on this subject. The oldest hominid fossils–belonging to species closer to us than to other apes–come from the eastern Sahara, Kenya, and Ethiopia. By 3.3 million years ago, hominids were in South Africa as well. These hominids lived in complicated landscapes, made up of woodlands and savannas. Gorillas today live in a very different sort of place: the forests of central and western Africa. Paleontologists have yet to find a fossil gorilla bone, so they have little to say about where their ancestors lived over the past seven million years. Were there once savanna gorillas? Were there once deep-jungle hominids?

And then there is the matter of where the lice live. Today, lice live on little islands of hair on an ocean of hairless human skin. They are clearly adapted to our relatively hairless bodies. The authors suggest that their results may mean that hominids were already losing hair 3.3 million years ago. The gorilla lice needed an empty ecological niche–pubic hair–that they could occupy in order to survive. If hominids had full-body hair, the lice that already lived on it might have been able to outcompete an invader.

This study is just the latest to demonstrate how much parasites can tell us about our past. While fossils remain hidden in the ground, while extinct species of hominids are no longer around to give us clues to our origins, our lice–and ulcer bacteria and tapeworms and other pathogens–are waiting for our questions. So when scientists call for the preservation of endangered parasites (pdf), don’t laugh–they want to preserve history.

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0 thoughts on “Question of the Day: How Do You Get Crabs From A Gorilla?”

Carl, I remember reading a bit about this in the NY Times’ Science Times about a year ago.. Was that your article?
Also, is there not a third type of lice we carry, which are adapted to gripping clothing?

Bob,
Yes, body lice (or clothing lice) are closely related to head lice. They are either considered to be two subspecies of Pediculus humanus, or two closely related species. The jury is still out on this. What is interesting about clothing lice is that of all the lice on the planet they are the only ones that lay eggs in something other than fur or feathers. Plus, they carry three deadly bacteria that don’t seem to be transmitted in nature by head lice. Although lab studies have shown that the head lice can transmit disease in controlled environments.

Brian S.,
Yes, it is possible that the switch went the other way, but that requires additional evolutionary steps. We haven’t sampled enough gorilla lice (or human pubic lice for that matter) yet in order to get a proper assessment of how much standing genetic variation there is in either species.

On the subject of parasites, the other parasite that has helped clarify some of human history is the mitochondrial DNA. Quite distinct and different from the autosomal DNA, and only inherited from (and passed on by) our mothers, it obvioulsy had a different origin from the rest of us. And it is now an integral and essential part of us!

I read somewhere that in the Middle Ages lice were considered an integral part of the human physique; perhaps in those days our ancestors had a better understanding of nature and our place in it.

The article is very interesting, but despite of the molecular clock (with all its problems), don’t you think that estimating 2 extintions is more than enough to explain lice evolution. It is mor pasimonius than having to infer a reinfestation in the Homo lineage.

I know the lice aren’t genes, but I can’t help but look at that tree and wonder if something akin to incomplete lineage sorting might also explain it. In other words, could the extinction of the Pthirus species have occurred within the chimp lineage and not prior to the chimp-human split? David, if this possibility is addressed in your paper, I apologize for asking before I have read it.

since black people basically have the same hair covering their entire bodies (which resemble pubic hair) is it possible for them to have pubic lice as well as head lice living together throughout their bodies? like ebony and ivory living together in harmony…

Great comments all. Two extinctions is more parsimonious, but isn’t supported by the data (branch lengths don’t fit under that scenario).

Incomplete lineage sorting doesn’t help us solve the problem either, unfortunately. And, yes we look at the extinction of Pthirus in chimps alone (the simplest solution). Again, that hypothesis doesn’t fit the genetic data.

Pubic lice can indeed be found on other parts of the body although this is uncommon and is not restricted to any ethnic group.

Two papers have proposed that we lost our hair in an “attempt” to thwart parasites (most recently, Pagel and Bodmer 2003), however if true, it seems to have backfired on us. We have twice the lice compared to gorillas and chimps.

Everyone is losing their hair. Hair has a life cycle. Each individual hair grows for awhile, then falls out. Typically a new hair grows.

That’s why your eyebrows (at least my eyebrows) don’t have to be trimmed. When they get to full length, they fall out. You don’t notice eyebrow hairs so easily – they’re small. But the hair off of your head tends to clog the shower drain.

I doubt lice have much to do with it. But what, exactly, do they eat? Dead skin?

All sucking lice feed solely on host blood. There is another group of lice (for example, lice of gophers) that do feed on dead skin. Neither of these louse groups, however, are known to cause hair loss.

An awful lot is being made in the popular reporting about the host shift that occurred about “3.3 million years ago”.

However, a look at the original paper shows the 95% confidence interval spans 2 – 6 million years ago, a pretty wide margin, and given the known vagaries of molecular dating techniques it would be nice to see a little more caution used in reporting the dates. Not to question the original study, of course, which looks solid considering the data and analyses.

It’s also worth noting that many humans are still pretty hairy, or would be if they didn’t wear clothes (which rub off much of the hair). Of course, that’s still “hair” rather than the more complex “fur”.

Quick question for the authors of the study: where did you get your samples? I’ve worked in several STD clinics and we don’t routinely (read: ever) sample the public lice populations we see there. We just try to kill em.

So where did you get your samples? And are you “recruiting” for more? (I knew a medical entymologist once who accosted me for putting a patient’s sample between two layers of stickytape when I could have simply put the buggers in ethanol. (Yeah. That’s a lot easier to ship.) Do you need docs to start helping you get more of these guys? Or do you have the problem, for lack of a better phrase, well in hand?

Even until very recently tribes in Central Africa have had sex with monkeys. There was even a profession which trained certain types of monkeys/apes to have sex with men. A touring caravan of trained monkeys moves from remote village to remote village. My father observed one such troupe himself in the 1970s, with young men lining up outside a hut about to take their turn.

Amongst young men bestiality is widely practised (chickens etc) and should be talked about more openly. At the moment political correctness prevents any discussion.

They always talk about butchering of animals, (sleeping in the same hollows etc) causing the transmission jump of HIV. Repeated sexual encounters between apes/monkeys and humans is far more likeley a method of transmission especially as its part of the behaviour pattern. Pubic lice from Gorrilas seems another part of the same pattern.

Given our genetic programming – to kill males and capture and sexually enslave females of our species and other hominids its not that surprising.

To respond to both Doug and Burt H:
To our knowledge, no one has examined Bonobo lice. We are trying to get some samples through collaborators; these lice would be excellent additions to future studies. Orangutans and Gibbons are not known to be parasitized by sucking lice. They either never had lice, or the louse populations on these primates went extinct.

We get most of our lice (Pediculus, Pthirus, and otherwise) from generous donations and long-term collaborations. We are ALWAYS looking to collaborate and we are ALWAYS in need of more samples. All author emails are available on the BMC Biology paper and everyone is more than welcome to contact us should they have specimens they would like to donate!

yeah, that was a total joke by the way… i happen to be in the armed services which is a very diverse workplace, and i only posted that comment to get a rise out of my shipmates, no harm intended, thanks for being good sports fellas…

I just read the very interesting original paper. I like the figure pointing head and pubic lice to Darwin. One thing that I don’t find clear from the paper is why the 3.3 million date could not mark an event within proto-gorilla populations. That way the lice could have jumped to humans more recently, and the 3.3 million host could be extinct.

This could be solved by something the authors point in their conclusions: finding old lineages in humans like it was found for head lice. If David Reed is still reading this, maybe he can tell us if there is any hint of distinct lineages in human pubic lice? Well, I think I know the answer: David Reed needs more pubic lice!

Stephen said, “That’s why your eyebrows (at least my eyebrows) don’t have to be trimmed. When they get to full length, they fall out.”
Oh yeah, what about Leonid Brezhnev…or Andy Rooney for that matter? Anyway, fascinating article.

The Wikipedia has been known to be wrong, but “Lice (singular: louse), also known as fly babies, (order Phthiraptera) are an order of over 3,000 species of wingless phthiraptra. They are obligate ectoparasites of every mammalian and avian order, with the notable exceptions of Monotremata (the duck-billed platypus and the echidna or spiny anteater) and Chiroptera (bats).?http://en.wikipedia.org/wiki/Lice

Do Orangutans and Gibbons have any type of lice that might be compared to sucking lice?

Wikipedia isn’t necessarily wrong in it’s definition of lice (although I’ve never heard “fly babies” before), but it does lack detail. Wikipedia also lumps information together regarding chewing lice and sucking lice. Here’s a link to another reference from the “Tree of Life” webpage; I think it provides more helpful information: http://tolweb.org/Phthiraptera

Organutans and Gibbons don’t have any lice all! Nothing! We can only hypothesize that either they had lice in the past and lost them (via a whole slew of possible reasons) or they never had them at all. Trying to reconstruct the evolutionary history of all primate lice will definitely help resolve this issue.

Gorilla and Human Pthirus diverged sometime between 1.84Mya and 5.61Mya.
Chimp and human head lice diverged sometime between 3.94Mya and 9.96Mya
The Pedicinus clade diverged sometime between 7.08 and 14.94Mya

There is also diversity among human pubic lice, which I known from personal experience. Because pubic lice survive better if they are hard to see, they have adapted to the color of the human they inhabit. In other words there are both black and white pubic lice. They may be the same species, but white people have have white lice and black people have black lice. Since it’s much easier to get rid of lice that stand out on your skin, I post this as a plea for more interracial liasons!

I noticed that in addition to the two louse extinction event that Dr. Reed identify’s in his paper, he posits several more in orangutans and gibbons. I am (perhaps naively) surprised by this frequency of parasite extinction when the hosts persist. Obviously parasites can suffer from their own diseases, but I am still somewhat surprised. Perhaps that is because the parasite population should be several orders of magnitude larger than the host population.

Upon further inspection, it appears that in the case of the two extinctions in the article, it was a case of competing populations, where Pediculus and Pthirus each won out in a different lineage. But that doesn’t explain why orangutans and gibbons would be louse-less.

As a moral human being I believe I have a right to know which one of my ancestors was screwing gorillas. Was it Orrorin tugenensis or was it Homo erectus? Can I be 99% sure that they are 95% sure they don’t know?

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Ed Yong is an award-winning British science writer. Not Exactly Rocket Science is his hub for talking about the awe-inspiring, beautiful and quirky world of science to as many people as possible.
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